Reference to a Satellite Positioning System or SATPS herein refers to a Global Positioning System (GPS), to a Global Orbiting Navigation System (GLONASS), and to any other compatible satellite-based system that provides information by which an observer's position and the time of observation can be determined.
The complete GPS system consists of 24 operational satellites and provides 24-hour, all-weather navigation and surveying capability worldwide.
The GPS satellites transmit at frequencies L1=1575.42 MHz and L2=1227.6 MHz modulated with two types of codes and with a navigation message. The two types of codes are the C/A-code and the P-code.
Two types of observables are of interest to users. One is the pseudo-range, which equals the distance between the satellite and the receiver plus small corrective terms due to clock errors, the ionosphere, the troposphere, and the multipath. Given the geometric positions of the satellites (satellite ephemeris), four pseudo-ranges are sufficient to compute the position of the receiver and its clock error. Pseudo-ranges are a measure of the travel time of the codes.
The second observable, the carrier phase, is the difference between the received phase and the phase of the receiver oscillator at the epoch of measurement. Receivers are programmed to make phase observations at the same equally spaced epochs. The receivers also keep track of the number of complete cycles received since the beginning of a measurement. Thus, the actual output is the accumulated phase observable at preset epochs.
(The above-referenced discussion is provided in the book "GPS Satellite Surveying", Second Edition, authored by Alfred Leick, and published by John Wiley & Sons, Inc. in 1995; pp 1-3).
The multipath errors originate with contamination of SATPS signals by delayed versions of these signals. For some applications using either pseudo-range or carrier phase observables, multipath is the dominant error source. The multipath disturbance at any location is highly dependent on the surrounding environment. The most direct approach for reducing this error is to select an antenna site distant from reflecting objects, and to design antenna/back plane combinations to further isolate the antenna from its surroundings. However, this approach suffers from several problems. Indeed, we may not have the luxury of putting an antenna in the open area. Also, there is a problem with design of antenna/back plane combinations: it is bulky, it increases the cost of the apparatus, and it is inconvenient.
Thus, what is needed is a technique which gives one an opportunity to assess the multipath properties at a given location, potentially in real time, in order to determine the code multipath error signal as a function of the delay of the multipath signal at the present site. This technique implemented in real time would allow one to roam about the selected site to find an optimal location which would introduce the least amount of multipath disturbance. This knowledge can be utilized in optimal placement of fixed antennas of base or reference stations.